# std::reverse_iterator

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class Iter > class reverse_iterator; |  |

`std::reverse_iterator` is an iterator adaptor that reverses the direction of a given iterator, which must be at least a LegacyBidirectionalIterator or model `bidirectional_iterator`(since C++20). In other words, when provided with a bidirectional iterator, `std::reverse_iterator` produces a new iterator that moves from the end to the beginning of the sequence defined by the underlying bidirectional iterator.

For a reverse iterator r constructed from an iterator i, the relationship &*r == &*(i - 1) is always true (as long as r is dereferenceable); thus a reverse iterator constructed from a one-past-the-end iterator dereferences to the last element in a sequence.

This is the iterator returned by member functions `rbegin()` and `rend()` of the standard library containers.

### Nested types

Type | Definition
`iterator_type` | `Iter`
`iterator_category` | std::iterator_traits<Iter>::iterator_category[1]
`value_type` | std::iterator_traits<Iter>::value_type[1]
`difference_type` | std::iterator_traits<Iter>::difference_type
`pointer` | std::iterator_traits<Iter>::pointer
`reference` | std::iterator_traits<Iter>::reference
| (until C++20)
Type | Definition
`iterator_type` | `Iter`
`iterator_concept` | std::random_access_iterator_tag if `Iter` models std::random_access_iterator std::bidirectional_iterator_tag otherwise
`iterator_category` | std::random_access_iterator_tag if std::iterator_traits<Iter>::iterator_category models std::derived_from<std::random_access_iterator_tag> std::iterator_traits<Iter>::iterator_category otherwise
`value_type` | std::iter_value_t<Iter>
`difference_type` | std::iter_difference_t<Iter>
`pointer` | std::iterator_traits<Iter>::pointer
`reference` | std::iter_reference_t<Iter>
| (since C++20)

1. ↑ 1.0 1.1 The definition is provided by the base std::iterator specialization until C++17.

### Data members

Member | Description
`Iter` `current` | the underlying iterator (protected member object)

### Member functions

(constructor) | constructs a new iterator adaptor (public member function)
operator= | assigns another iterator adaptor (public member function)
base | accesses the underlying iterator (public member function)
operator*operator-> | accesses the pointed-to element (public member function)
operator[] | accesses an element by index (public member function)
operator++operator++(int)operator+=operator+operator--operator--(int)operator-=operator- | advances or decrements the iterator (public member function)

### Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20) | compares the underlying iterators (function template)
operator+ | advances the iterator (function template)
operator- | computes the distance between two iterator adaptors (function template)
iter_move (C++20) | casts the result of dereferencing the adjusted underlying iterator to its associated rvalue reference type (function)
iter_swap (C++20) | swaps the objects pointed to by two adjusted underlying iterators (function template)
make_reverse_iterator (C++14) | creates a std::reverse_iterator of type inferred from the argument (function template)

### Helper templates

template< class Iterator1, class Iterator2 > requires (!std::sized_sentinel_for<Iterator1, Iterator2>) inline constexpr bool disable_sized_sentinel_for <std::reverse_iterator<Iterator1>, std::reverse_iterator<Iterator2>> = true; |  | (since C++20)

This partial specialization of `std::disable_sized_sentinel_for` prevents specializations of `reverse_iterator` from satisfying `sized_sentinel_for` if their underlying iterators do not satisfy the concept.

### Possible implementation

Below is a partial implementation focusing on the way the inner iterator is stored, calling std::prev only when the content is fetched via operator*.

    template<class It>
    class reverse_iterator
    {
    protected:
        It current = It();
    public:
        reverse_iterator() = default;
        constexpr explicit reverse_iterator(It itr) : current(itr) {}
        template<class U>
            requires (!std::is_same_v<U, It> && std::convertible_to<const U&, It>)
        constexpr explicit reverse_iterator(const U& other) : current(other.base()) {}

        constexpr decltype(auto) operator*() const
        {
            return *std::prev(current); // <== returns the content of prev
        }

        constexpr reverse_iterator& operator++() { --current; return *this; }
        constexpr reverse_iterator operator++(int) { auto tmp = *this; ++(*this); return tmp; }

        constexpr reverse_iterator& operator--() { ++current; return *this; }
        constexpr reverse_iterator operator--(int) { auto tmp = *this; --(*this); return tmp; }

        constexpr It base() const { return current; }

        // Other member functions, friend functions, and member typedefs are not shown here.
    };

### Notes

`std::reverse_iterator` does not work with iterators whose dereference returns a reference to a member of *this (so-called “stashing iterators”). An example of a stashing iterator is MSVC STL's `std::filesystem::path::iterator`.

### Example

Run this code

    #include <cstddef>
    #include <iostream>
    #include <iterator>

    template<typename T, std::size_t SIZE>
    class Stack
    {
        T arr[SIZE];
        std::size_t pos = 0;
    public:
        T pop()
        {
            return arr[--pos];
        }

        Stack& push(const T& t)
        {
            arr[pos++] = t;
            return *this;
        }

        // we wish that looping on Stack would be in LIFO order
        // thus we use std::reverse_iterator as an adaptor to existing iterators
        // (which are in this case the simple pointers: [arr, arr + pos)
        auto begin() { return std::reverse_iterator(arr + pos); }
        auto end() { return std::reverse_iterator(arr); }
    };

    int main()
    {
        Stack<int, 8> s;
        s.push(5).push(15).push(25).push(35);
        for (int val : s)
            std::cout << val << ' ';
        std::cout << '\n';
    }

Output:

    35 25 15 5

### See also

make_reverse_iterator (C++14) | creates a std::reverse_iterator of type inferred from the argument (function template)
iterator (deprecated in C++17) | base class to ease the definition of required types for simple iterators (class template)

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